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Modern physics fully accepts the concept of wave-particle duality in case of light and other electromagnetic radiation. The phenomena such as interference, diffraction, and polarization can only be explained when light is treated as a wave whereas the phenomena such as the photoelectric effect, line spectra, and the production and scattering of x rays demonstrate the particle nature of light. In this article, in order to show the particle nature of light, I have discussed the photoelectric effect along with the necessary equations.

The photoelectric effect is defined as a phenomenon in which the emission of electrons occurs when a beam of light strikes a metal or a cathode surface. For the emission of electrons to take place, the frequency of incident light is required to be greater than a certain minimum value. This value is known as the threshold frequency. The threshold frequency depends on the metal or material of the cathode. For most of the metals, threshold frequency is in the ultraviolet range (wavelengths between 200 nm to 300 nm). If the intensity of light (I) is increased while keeping the frequency same, more electrons are emitted per unit time. Thus, photocurrent is directly proportional to the intensity of light.

The photoelectric effect was correctly explained by the world-famous physicist, Albert Einstein, in the year 1905. It is interesting to know that Einstein was awarded the Nobel Prize in Physics 1921 for his work on photoelectric effect. Einstein figured out that a beam of light comprises of small packages of energy known as photons or quanta. The energy of a photon (E) is equal to the product of the Planck’s constant (h) and the frequency of a photon (f). Frequency (f) of photon = speed of light (c)/wavelength of photon (λ).